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Schrödinger’s cat, the thought experiment, makes it seem like as if measurement can cause a system to stop being in a superposition of states and become either one of the states (collapsed).

So does a system always exist in a superposition? In this sense, do things in the macroscopic world have a wavefunction? Is it because of the size of the everyday object, so it won't behave so much like an electron? Theoretically, if the Planck constant is to be bigger, everyday object would start behaving more like particles in a quantum scale?

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In principle everything exists in a superposition of states. However everything interacts with it's environment, and this collapses the superposition through a mechanism called decoherence. In the particular case of Schrodinger's cat there is a brief discussion of this here. Also search this site for decoherence as there have been lots of questions about it.

For any system, like the cat, there is an associated decoherence time that tells you approximately how long it takes the superposition to collapse. For objects on the atomic scale this can be quite long, but for macroscopic objects the decoherence time is extremely short and we can never observe these objects to be in a superposition of states.

Someone has actually done an experiment to prepare a virus in a superposition of states . Because a virus is so much smaller than a cat the superposition can be observed. Luboš Motl has written a fascinating article about this on his blog.

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Thanks! The links are very helpful. –  user8302 Jan 23 '13 at 10:11
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I'm pretty sure Planck's constant only refers to the size of quantum scale objects. If it were larger, all it would mean is that Newtonian physics would only apply at a larger scale.

And Schrodinger's cat only means that until observed, we cannot be sure of the state. It has a state, whether we know it or not. What we are not able to know is which state it is in. Observing something doesn't set the state, it just stops us from needing to account for multiple possible stats. And quantum mechanics is based partly around not knowing beforehand which state it will be in.

Macroscopic objects do have a wavefunction, actually. Since for any wave, E=hc/lambda, lambda = hc/E. You can then substitute in the formula for E to get the wavefunction of that object. Most objects simply do not have a visible wavelength, but a good example of adding energy to an object to shift it into visible light is heating up metal until it glows.

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